That’s a striking reversal from 2008, when Obama won independent voters, who made up 29 percent of the electorate, by eight points over Sen. John McCain of Arizona.

And if Romney’s large margin among independents holds, it will be a break not just from 2008 but also from 2000 and 2004. In 2000, Texas Gov. George W. Bush won independents by 47 percent to 45 percent over Vice President Al Gore. Four years later, Bush and Sen. John F. Kerry of Massachusetts essentially split unaffiliated voters, according to exit polls — 48 percent for Bush to 49 percent for Kerry. (Independents made up 27 percent of the vote in 2000 and 26 percent in 2004.)

So, what gives? Why is Obama — at least according to the Post-ABC data — having so much trouble with independents?

The answer lies in the fact that most independents are not, well, independent. Of all the likely voters who called themselves independents in nine days of the Post-ABC tracking poll, fully three-quarters (75 percent) — said they tend to lean toward one party or the other. (The remainder are known as “pure” independents.)

And it’s among those shadow partisans that Obama is struggling. Ninety-two percent of Republican-leaning independents said they plan to support Romney, while 84 percent of Democratic-leaning independents are backing Obama.

It’s not just in the head-to-head matchup that the difference between GOP-leaning and Democratic-leaning independents is visible. Among all registered voters, 69 percent of Republican-leaning independents say they are following the election closely while just 49 percent of Democratic-leaning independents say the same. (Just more than four in 10 — 41 percent — of pure independents say they are closely following the election.)

That gap between partisan-leaning independents was just nine points in September but has now grown to a 20-point edge this month as the election draws near.

By way of comparison, in Post-ABC polling conducted in October 2008, 62 percent of Democratic-leaning independents said they were closely following the election while 60 percent of Republican-leaning said the same.

Among independents who say they are “absolutely certain” to vote, 87 percent of Republican-leaning independents express that sentiment, compared with 81 percent of Democratic-leaning independents.

What all those numbers mean is that among independent voters — who tend to be less likely to turn out, even in a presidential election, than partisans — Romney has a clear edge.

Now, several caveats are worth noting.

First, while the election is national in scope, it will be decided in a handful of swing states, including Virginia, where Post polling released Sunday showed Obama with a four-point edge.

Second, enthusiasm among independents can be a fleeting thing — as shown by the movement in the numbers among GOP-leaning independents over the past few weeks.

Third, even Republicans acknowledge that Obama’s turnout operation is the best that has ever been built, meaning (a) the incumbent’s campaign will find every Democratic partisan there is in a swing state and (b) it will work hard to contact and energize those Democratic-leaning independents in the final eight days of the campaign.

Still, Romney’s wide lead among independents in Post-ABC tracking polling is a remarkable finding, given the narrow margins between the two men overall. (Romney polled 49 percent to Obama’s 48 in Sunday’s Post-ABC tracking survey.) And, if Romney wins the election Nov. 6, he will almost certainly have independents loosely affiliated with Republicans to thank for his victory.